Hi,
Assuming you are on QF 93/94 to MEL;
http://www.qantas.com.au/travel/airl...eles/global/en
you will need to get from T3 ( Alaska ) to tom bradley intl terminal ( QF) and reclear security.
Even though the terminals are beside each other the security clearance will take time, QF will board with at least 40mins to go and the gate will probably close at T-20 or T-10.
I would phone Qantas and get on the earlier flight from SEA-LAX. I do not know if getting the earlier flight would have an impact on your connecting time at SEA from your flight from JNU.
Regards
TBS